made a 13mm square socket from a piece of 1in hex to change the rear diff oil in my Subaru. Glad I did too, foul black stuff came out. Did the front as well (not quite as disgusting) and the CVT is next weekend.
used my lathe to face both ends of the hex (one from the scrap yard, one from me hacksawing the chunk off), then cut a groove to the diagonal distance across the hex (18mm?). Hacksawed off the corners and squared things up on my "mill". Worked a treat!